Combined supply and blanking circuit for eliminating supply voltage variations of a color cathode-ray tube

ABSTRACT

A CIRCUIT FOR ELIMINATING THE EFFECTS OF SUPPLY VOLTAGE VARIATIONS ON A CATHODE-TAY TUBE. THE CATHODES AND CONTROL GRIDS ARE CONNECTED TO ONE TERMINAL OF AN UNSTABILIZED POWER SUPPLY THROUGH LOW VALUE RESISTORS. IN ADDITION, THE CATHODES AND CONTROL GRIDS ARE CONNECTED TO THE OTHER TERMINAL THROUGH A HIGH IMPEDANCE, SUCH AS TRANSISTORS. THEREFORE, ANY CHANGE IN THE SUPPLY VOLTAGE EQUALLY EFFECTS BOTH ELECTRODES.

United States Patent Inventor Marcel Rognon [56] References Cited 11661116, France umrso STATES PATENTS 3 2 2 2,520,728 8/1950 Kline etal 315/13 Id 1971 2,845,573 7/1958 Macovski... 315/13(co) e "hi C 6 2,906,917 9/1959 Simpson 315/13(CG) 3,283,065 11/1966 Boekhorst.. 315/13 co x Priority My, 1968 3,301,943 1/1967 Hansen 315/13 co)x Fun Primary ExaminerRodney D Bennett, Jr. 1578.06 Assistant Examiner-Malcolm F. Hubler Attorney-Frank R. Trifari COMBINED SUPPLY AND BLANKING CIRCUIT FOR ELIMINATING SUPPLY VOLTAGE VARIATIONS OF A COLOR CATHODE-RAY TUBE 7 Claims, 1 Drawing Fig.

U.S. Cl

Int. Cl Field of Search ABSTRACT: A circuit for eliminating the effects of supply voltage variations on a cathode-ray tube. The cathodes and 315/13, control grids are connected to one terminal of an unstabilized 3 l5/ 1 4 power supply through low value resistors. In addition, the 1101 j 29/50 cathodes and control grids are connected to the other terminal 3 15/ l3, l3 through a high impedance, such as transistors. Therefore, any

(CG), [4 change in the supply voltage equally effects both electrodes.

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FLYBACK PULSES 50 VE RTICAL 1 FLYBACK PULSES COMBINED SUPPLY AND 'BLANKING CIRCUIT FOR ELIMINATING SUPPLY VOLTAGE VARIATIONS OF A COLOR CATHODE-RAY TUBE The invention relates to a television picture display device comprising a circuit for maintaining the bias of electrodes of a picture display tube constant, which picture display tube includes at least a cathode, a wehnelt electrode and a further electrode which are connected to a supply circuit.

Such a television picture display device is known from German Pat. Specification 1,175,356, wherein a constant bias of some electrodes of the picture display tube upon a variation of the contrast control has been achieved.

An object of the invention is to obtain a constant bias of the electrodes when supply voltage variations occur.

To this end a television picture display device of the kind described in the preamble according to the invention is characterized in that both the cathode and the wehnelt electrode are each connected to a first terminal of one and the same voltage supply source through an impedance which is low for voltage variations of the voltage supply source relative to the impedance .between each of these electrodes and a second terminal of this voltage supply source, so that these voltage variations at the wehnelt electrode and the cathode are substantially equal.

If according to a further embodiment of the invention, both the wehnelt electrode and the cathode are each connected to the collector and the load resistor of a transistor and the load resistors are connected at the other end to the first terminal of the supply source and the transistors are connected at the other end to the second terminal of the supply source, the suppression of the beam current in the flyback periods may take place by applying a blanking signal to the control electrode of, for example, the transistor connected to the wehnelt electrode.

In the so-called R, G and B control in the case of a color picture display tube, the three wehnelt electrodes may be interconnected and controlled by the same blanking signal.

R, G and B amplifiers can then preferably be used so that it i is possible to control the picture tube in such a manner that the correct ratio of the amplitude of the color signals is maintained throughout the characteristic of the three electron guns. Such a device has been described in French Pat. Specification No. 1,564,066 granted to the Applicant and entitled Procede et dispositif de reglage damplificateurs de signaux de couleur pour recepteurs de television corresponding to U.S. Pat. application No. 804,521, filed Mar. 5, I969.

Furthermore there is still a problem consisting in blanking unwanted signals which may control the picture tube. In the SECAM color television system, color identification signals occur during the field-blanking period of a chrominance video signal which identification signals are each preceded by an interruption of the chrominance subcarrier, while noise may occur during the line blanking period.

After demodulation and matrixing, the color identification signals present in the SECAM-system in the channels for the red and the blue color signals are not troublesome because their polarity is such that they attempt to block the corresponding electron guns. The polarity of the identification signals for the green color signals in the channel is, however, reversed. It is therefore necessary for the electron gun controlled by the green color signals to be blocked during the field blanking period.

In a further elaboration of the invention the three electron guns may best be blocked all at the same time during the blanking periods. lnterferences as a result of noise signals during the line and field blanking periods and having thesame polarity as the picture signal can then no longer influence the three display systems.

In order that the invention may be readily carried into effect, it will now be described in detail by way of example with reference to the accompanying diagrammatic drawing, which only shows one F lGURE, and a nondetailed circuit diagram of a device according to the invention.

In this FIGURE, the base of an NPN transistor 4 is connected to ground through a parallel arrangement of a resistor 3 and a capacitor 2. Field flyback pulses are applied through an input 1 to the base of this transistor 4.

The collector of the transistor 4 is fed through a resistor 5 which is connected to a positive terminal 49 of a voltage source Vbl the negative terminal of which is connected to ground. The collector of the transistor 4 is also connected to an output terminal 12 through a series arrangement of a resistor 6 and a capacitor 7.

The base of an NPN transistor the collector of which is fed from the positive terminal Vbl through a resistor 14 is connected to the collector of the transistor 4 through a resistor 9. The emitter of the transistor 15 is directly connected to the emitter of the transistor 4 and is connected to ground through a common emitter resistor 10. The collector of the transistor 15 is furthermore connected through a resistor 11 to an output terminal 13.

A potential divider formed by a series arrangement of a resistor 21 and a Zener diode 22 is provided between the terminal +Vbl and ground. The junction between this resistor 21 and this Zener diode 22 is connected to the end of a series arrangement of diodes 17, 18, 19 and 20. The other end of this series arrangement of diodes is connected to ground.

The interconnected cathodes of the diodes 17 and 18 are connected through a resistor 8 to the collector of the transistor 4. The interconnected anodes of the diodes l9 and 20 are connected through a resistor 16 to the collector of transistor 15.

The junction between the diodes l8 and 19 is connected to an output terminal 23.

The three cathodes of a color picture display tube 24 having three electron guns are controlled by the collectors of three transistors 25, 27 and 29 which form the output stages of R, G and B color signal amplifiers. Three resistors 26, 28 and 30 which are connected to the positive terminal 47 of a voltage source Vb2, the negative terminal 48 of which is connected to ground, form the load resistors of these transistors 25, 27 and 29.

The collector of an NPN transistor 34 applies blanking signals to the three interconnected control grids (wehnelt electrodes) of the picture tube 24. The collector of the transistor 34 is connected through a resistor 36 to the positive terminal of the voltage supply source Vb2. The emitter of the transistor 34 is connected to ground through a variable resistor 35. The base of the transistor 34, which receives a bias voltage through a potential divider 31, 33 connected between the positive terminal of the supply source Vbl and ground, is connected on the one hand through a resistor 32 to the collector of the transistor 15 and on the other hand through a series arrangement of.a resistor 51 and a diode 52 to an input 50 of which line flyback pulses are applied.

The three screen grids of the picture tube 24 are fed by the potentiometers 39, 42 and 45 which are connected between the positive terminal 46 of a supply source V123 and the negative terminal 47. The negative terminal 47 of the supply source Vb3 is connected to the positive terminal of the source Vb2. Series resistors 38, 41 and 44 are provided between the adjustable tappings on the potentiometers 39, 42 and 45 and the three screen grids. The screen grids are decoupled with respect to the junction +Vb2, Vb3 of the supply sources Vb2 and Vb3 through capacitors 37, 40 and 43, respectively.

The operation of the circuit arrangement is as follows:

The electronic switch or blanking pulse generator formed by the transistors 4 and 15 and the double limiter circuit formed by the diodes 17, 18, 19 and 20 constitute a noise suppression circuit which is the subject of a French Pat. Application filed by the Applicant at the same time with the present No. 1,587,632 and entitled Dispositif anti-perturbations pour recepteurs de television en couleur."

lf no field flyback pulses are present at the terminal 1 the transistor 4 is cut off and the transistor 15 is saturated. The four diodes 17, 18, 19 and 20 are then blocked, while the signal at the terminal 23, which in a SECAM receiver is connected to the output of the frequency discriminator for the blue color difierence signal, does not exert any influence on the operation of this discriminator.

The positive field flyback pulses at the terminal 1 render the transistor 4 conducting and cut off the transistor 15. Under the influence of the signal at the terminal 23 the color identification signal occurring in the output signal of the blue color difference signal discriminator is limited between ground potential and the positive potential determined by the Zener diode 22. In addition, the signal at the output terminal 12 causes the color identification signals in the red" channel to be blanked, while furthermore the signal at the output terminal 13 in the blue" channel ensures a constant level of limitation for the color identification signals.

If no pulses are applied to the input 1 and if the transistor 15 is thus fully saturated, its collector potential is substantially equal to ground potential and the base of the transistor 34 receives a bias voltage from the potential divider formed by the resistor 31 and the equivalent resistance of the parallel arrangement of the resistors 32 and 33. This condition also oc curs in the absence of line flyback pulses at the input 50. The transistor 34 then conducts current and the voltage drop caused thereby across the collector resistor 36 ensures the rest bias of the wehnelt cathode trajectories of the picture tube 24. This voltage range is adjustable with the aid of the emitter resistor 35. The cathodes of the picture tube 24 then have the potential of the terminal 47, because the transistors 25, 27 and 29 do not draw current in case of the rest bias.

The cutoff point of each electron gun must previously be adjusted by the adjustment of the voltage at the screen grids of the picture display tube 24 by means of the potentiometers 39, 42 and 45, respectively.

The field flyback pulses applied to the terminal 1 and the line flyback pulses applied to the terminal 50 cause saturation of the transistor 34 whose collector voltage then becomes ground potential, the voltage between the cathode and the wehnelt electrode of each electron gun becoming maximum and the electron guns being blocked.

The base of the transistor 34 is fed from a stabilized voltage source Vbl which is usually present elsewhere in the television set. The voltage supplied by this voltage source is comparatively low. The second grids of the picture tube are fed by a stable voltage supplied by the line deflection stage. The negative terminal of the voltage source Vb3 which supplies this voltage is connected to the positive terminal of the nonstabilized voltage source Vb2 which supplies the collector of the transistor 34 and also the collector of the transistors 25, 27 and 29.

The transistor 34 thus determines two bias voltages of the first grids of the picture tube, namely those at which the base of the transistor 34 receives or does not receive frame and line flyback pulses. The special manner according to the invention in which the transistors 34, 25, 27 and 29 are connected in series with a load resistor to the same supply source ensures that the voltages at the three electrodes of each electron gun of the picture tube always substantially vary to the same extent whatever the fluctuations of the nonstabilized voltage source may be. In fact, the transistors 34, 25, 27 and 29 form impedance for such fluctuations which are high relative to those of their collector resistors 36, 26, 28 and 30, respectively.

Consequently, supply voltage variations of the supply source Vb2 are substantially passed on both at the cathodes and the wehnelt electrodes of the picture display tube 24, while these variations also appear substantially at the screen grids through the floating supply source Vb2 The different electrodes therefore do not undergo any voltage variations relative to one another so that no color errors can appear as a result thereof.

If a phase shift occurs in the phase discriminator of the line scanning stage, the line flyback pulses may show a shift which would result in an incorrect picture blanking. To eliminate this drawback the effect of the delay caused by the storage of mmority charge carriers in the transistor 34 is used to extend the duration of the trailing edges ofthe line flyback pulses.

In case of a possible breakdown between the screen grids and the anode brought to a very high voltage, the decoupling capacitors 37, 40 and 43 may furthermore limit the effect of such a breakdown.

Without passing beyond the scope of the present invention, it is of course possible to make certain changes in the embodiments described, namely by replacing equivalent technical means. Thus the circuit arrangement for the signal-dependent voltages according to the invention may alternatively be used in a monochrome receiver which must satisfy strict requirements. Furthermore, electron tubes or other suitable active elements may be used instead of transistors. The electrodes of the picture display tube may also be controlled by signals other than those given in the embodiment.

lclaim:

1. A television display circuit for operation from a power supply, comprising a cathode-ray tube including a cathode and a wehnelt electrode; first and second terminals coupled to said supply; means for eliminating the effect of supply voltage variations on said tube including first and second low impedance resistor means for coupling said cathode and said wehnelt electrode respectively to said first terminal, and first and second high impedance means each having two conduction and a control terminal, said conduction terminals of said first high impedance means being coupled to said cathode and second terminals respectively, said conduction terminals of said second high impedance means being coupled to said wehnelt and second terminals respectively, said first high impedance coupled to receive a video intensity signal, said second high impedance control terminal being coupled to receive a blanking pulse.

2. A circuit as defined in claim 1 wherein said tube further comprises a further electrode and another low impedance means for coupling said further electrode to said first terminal.

3. A circuit as defined in claim 2 further comprising a stabilized power supply coupled between said first terminal and said further electrode.

4. A circuit as defined in claim 1 wherein said high impedance means comprises a transistor having emitter, base and collector electrodes, said bases being said control terminals, and said emitters and collectors being said conduction terminals.

5. A circuit as defined in claim 4 further comprising a blanking pulse generator coupled to said base of said second high impedance means transistor.

6. A circuit as defined in claim 5 further comprising a variable resistor coupled between said second terminal and said emitter of said second high impedance means transistor.

7. A circuit as defined in claim 5 wherein the amplitude of the blanking pulse is sufficient to saturate said second high impedance transistor. 

